The present invention relates generally to imaging systems. More particularly, the present invention relates to the cooling of x-ray windows in x-ray tubes.
Electron beam generating devices, such as x-ray tubes and electron beam welders, operate in a high temperature environment. In an x-ray tube, for example, the primary electron beam generated by the cathode deposits a very large heat load in the anode target to the extent that the target glows red-hot in operation. Typically, less than 1% of the primary electron beam energy is converted into x-rays, while the balance is converted to thermal energy. This thermal energy from the hot target is radiated to other components within the vacuum vessel of the x-ray tube, and is removed from the vacuum vessel by a cooling fluid circulating over the exterior surface of the vacuum vessel. Additionally, some of the electrons back scatter from the target and impinge on other components within the vacuum vessel, causing additional heating of the x-ray tube. As a result of the high temperatures caused by this thermal energy, the x-ray tube components are subject to high thermal stresses which are problematic in the operation and reliability of the x-ray tube.
Typically, an x-ray beam generating device, referred to as an x-ray tube, comprises opposed electrodes enclosed within a cylindrical vacuum vessel. The vacuum vessel is typically fabricated from glass or metal, such as stainless steel, copper or a copper alloy. As mentioned above, the electrodes comprise the cathode assembly that is positioned at some distance from the target track of the rotating, disc-shaped anode assembly. Alternatively, such as in industrial applications, the anode may be stationary.
The target track, or impact zone, of the anode is generally fabricated from a refractory metal with a high atomic number, such as tungsten or tungsten alloy. A typical voltage difference of 60 kV to 140 kV is maintained between the cathode and anode assemblies to accelerate the electrons. The hot cathode filament emits thermal electrons that are accelerated across the potential difference, impacting the target zone of the anode at high velocity. A small fraction of the kinetic energy of the electrons is converted to high energy electromagnetic radiation, or x-rays, while the balance is contained in back scattered electrons or converted directly into heat within the anode. The x-rays are emitted in all directions, emanating from the focal spot, and may be directed out of the vacuum vessel.
In an x-ray tube having a metal vacuum vessel, for example, an x-ray transmissive window is fabricated into the metal vacuum vessel to allow the x-ray beam to exit at a desired location. After exiting the vacuum vessel, the x-rays are directed to penetrate an object, such as human anatomical parts for medical examination and diagnostic procedures. The x-rays transmitted through the object are intercepted by a detector and an image is formed of the internal anatomy. Further, industrial x-ray tubes may be used, for example, to inspect metal parts for cracks or to inspect the contents of luggage at airports.
Since the production of x-rays in an x-ray tube is by its nature a very inefficient process, the components in x-ray generating devices operate at elevated temperatures. For example, the temperature of the anode focal spot can run as high as about 2700.degree. C., while the temperature in the other parts of the anode may range up to about 1800.degree. C. Additionally, other components of the x-ray tube must be able to withstand the high temperature exhaust processing of the x-ray tube, at temperatures that may approach approximately 450.degree. C. for a relatively long duration.
To cool the x-ray tube, the thermal energy generated during tube operation must be radiated from the anode to the vacuum vessel and be removed by a cooling fluid. The vacuum vessel is typically enclosed in a casing filled with circulating, cooling fluid, such as dielectric oil. The casing supports and protects the x-ray tube and provides for attachment to a computed tomography (CT) system gantry or other structure. Also, the casing is lined with lead to provide stray radiation shielding.
The cooling fluid often performs two duties: cooling the vacuum vessel, and providing high voltage insulation between the anode and cathode connections in the bipolar configuration. The performance of the cooling fluid may be degraded, however, by excessively high temperatures that cause the fluid to boil at the interface between the fluid and the vacuum vessel and/or the transmissive window. The boiling fluid produces bubbles within the fluid that may allow high voltage arcing across the fluid, thus degrading the insulating ability of the fluid. Further, the bubbles may lead to image artifacts, resulting in low quality images. Thus, the current method of relying on the cooling fluid to transfer heat out of the x-ray tube may not be sufficient for new, higher power x-ray tubes.
Similarly, excessive temperatures can decrease the life of the transmissive window, as well as other x-ray tube components. Due to its close proximity to the focal spot, the x-ray transmissive window is subject to very high heat loads resulting from thermal radiation and back scattered electrons. These high thermal loads on the transmissive window necessitate careful design to insure that the window remains intact over the life of the x-ray tube, especially in regard to vacuum integrity.
The transmissive window is an important hermetic seal for the x-ray tube. The high heat loads cause very large cyclic stresses in the transmissive window and can lead to premature failure of the window and its hermetic seal. Further, as mentioned above, direct contact with the cooling fluid can cause the fluid to boil as it flows over the window. Also, direct contact with a window that is too hot can cause degraded hydrocarbons from the fluid to become deposited on the window surface, thereby reducing image quality. Thus, the conventional method of cooling the transmissive window by simple immersion in a flow of oil may not be satisfactory.
The greatest localized heating of the x-ray window is due to back scattered electrons from the target impacting the window. The conventional method of providing cooling to the x-ray window is by a flow of the dielectric oil that is pumped through the casing of the x-ray tube assembly. As x-ray tubes become more powerful, this method of cooling has become inadequate. New techniques in x-ray computed tomography, such as, fast helical scanning, require vastly more powerful x-ray tubes. One proposed approach includes a device to electromagnetically deflect the back scattered electrons away from the window. This approach can be very difficult to implement and control and also causes greater heat loads on other components within the x-ray tube vacuum vessel.
As mentioned above, x-ray transmissive windows in metal-framed x-ray tubes can receive enormous heat fluxes due to thermal radiation and back scattered electrons from the anode. In metal-framed x-ray tubes, the transmissive window is typically made of a low atomic number material, such as, beryllium, aluminum, or titanium. Due to its close proximity to the x-ray focal spot, the x-ray window is subject to very high thermal loads and stress. The window joint integrity is, therefore, the weakest link in the sustainable hermeticity of the vacuum enclosure. Consequently, it is vital to provide adequate cooling to the x-ray window to ensure its structural and functional integrity over the life of the x-ray tube.
The material that forms the window (e.g., beryllium) is typically joined to the metal vacuum enclosure by brazing, soldering, welding, or some combination. In a typical application, beryllium is brazed into a copper carrier which is itself brazed into the steel vacuum vessel of an x-ray tube insert. The copper acts as a conduction heat sink for the beryllium and matches the thermal diffusivity and expansion characteristics.
Generally, the vacuum vessel and window are cooled by a bulk flow of dielectric oil, or similar coolant. However, as new, more powerful, x-ray tubes are developed, this simple style of cooling will prove to be inadequate. As such, novel techniques are required to ensure the survivability of the window.
Thus, there is a need for an apparatus which provides adequate cooling for x-ray transmissive windows such as those found in metal-framed x-ray tubes. Further, there is a need for an apparatus which provides heat dissipation at the junction of the x-ray window braze joint.